Fractures
Proximal Phalanx
Basic Information 
Epidemiology
Species, Age, Sex
• Osteochondral fractures involving the proximal dorsal rim of the proximal phalanx are common in racehorses.
• Osteochondral fractures involving the proximopalmar/plantar aspect of P1 are seen in young athletic horses and are classified as type 1 when they are avulsed from the axial portion of proximal P1 and are mostly articular. Type 2 lesions are abaxial, larger, and have limited articular cartilage and are most commonly seen in the hindlimbs.
• Diaphyseal fractures are seen in horses of all ages and are typically the result of trauma.
Clinical Presentation
History, Chief Complaint
• Osteochondral fractures involving the proximal dorsal rim of P1 typically present with a history of several days of moderate lameness that may improve with rest.
• Type 1 osteochondral fractures involving the proximopalmar/plantar aspect of P1 typically present for mild lameness that may only be evident at higher speeds.
• Type 2 lesions are often found only on radiographic evaluation, and most do not cause lameness.
• Horses with diaphyseal fractures most often have a history of trauma or acute injury.
Etiology and Pathophysiology
• Osteochondral fractures involving the proximal dorsal rim of the P1 are seen most commonly in racehorses and result from hyperextension of the metacarpophalangeal joint with impact on the dorsal region of the metacarpus.
• Osteochondral fractures involving the proximopalmar/plantar aspect of P1 were thought to be the result of osteochondrosis; however, recent evidence supports a fracture etiology.
• Diaphyseal fractures result from compressive and torsional forces transmitted from the sagittal ridge of the cannon bone through the sagittal groove propagating distally.
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